1. Field of the Invention
The present invention relates generally to gas analyzers and, more specifically, to microfabricated gas chromatography for trace analysis.
2. Description of the Prior Art
Ever since its inception, the standard in volatile mixture separations for analysis has been gas chromatography (GC). Traditional GC involves injecting the analyte mixture into a tubular column which is coated with a stationary phase or packed with a relatively inert porous material that is itself coated with a stationary phase. A common configuration is a wall coated fused silica capillary column. Depending on the carrier gas flow conditions in the column, the analyte boiling points and relative affinities for and interaction with the stationary phase, the different components of the analyte mixture elute from the column at various times (retention times). As the chemicals elute or exit the end of the column, they are detected and identified by a detector with the result displayed as a chart known as a chromatograph—response (y-axis) versus retention time (x-axis). In short, the chromatograph provides a spectrum of peaks representing the separated analytes present in an injected mixture eluting from the column at different times. Both retention times and the order of peaks will be constant for similar conditions and will yield a quick identification of the components. The process can be tuned for improved resolution and speed with temperature and pressure programming.
Quantitative analysis can also be carried out as the area under a peak is proportional to the amount of analyte present. By integrating and thus calculating the area under the peak, the concentration of an analyte in the original sample can be determined. In general, substances that vaporize below 300° C. (and therefore are stable up to that temperature) can be measured quantitatively.
Typical gas chromatographs consist of an injector system, a carrier gas supply, a capillary column coated with a polymeric stationary phase, a detector, and the associated control electronics for sampling, heating, and acquisition of data (FIG. 1). Conventional GC systems are expensive bench top instruments that require high power and offer typical analysis times measured in tens of minutes. Recently, microfabrication techniques have been applied to analytical systems to develop miniaturized systems that could allow analytical systems to be fabricated with sizes comparable to a wrist watch. Micromachining allows batch-fabricated devices with high quality control and the potential for inexpensive portable systems that consume minimal power. Furthermore, the thermal mass of microfabricated devices is small and direct heating allows rapid thermal ramping with much faster rates than in typical gas chromatographs, allowing a reduction in analysis times. These attributes make these devices attractive for a number of applications involving on-site monitoring of environmental samples.
When an analyte is injected into a GC column, it is normally vaporized just prior to entering the column. Depending on the injection method, fluidic design, flow conditions, and vaporization efficiency, a vapor band of analyte enters the column with some finite time width. In an ideal column, the band would retain its original time width throughout the length of the column. However, undesirable band broadening processes are prevalent in all commercial columns leading to a loss in chromatography separation power. Some features that lead to band broadening processes include non-uniform stationary phase coatings (eddy diffusion), diffusion limited analyte processes (resistance to mass transfer between mobile and stationary phases), and longitudinal diffusion processes that derive from high concentration areas (center of band) moving into low concentration areas (edge of band). In conventional capillary columns, polymer stationary phase coatings are coated by filling the column with a solution of polymer, capping one end, and pulling at a reduced pressure on the other end to slowly remove solvent from the column leaving behind a polymer coating. In newer microfabricated columns, the inside surfaces of the column may not be smooth and wherever there are sharp discontinuities such as a corner, stationary phase polymer material can undesirably pool during the deposition process or during subsequent movements that can occur at elevated temperatures.